As electrical systems age, defects such as: cavities inside of insulating materials; thinning of insulation in motor and transformer windings; contamination across insulating surfaces; incorrect voltage to ground spacing; etc., can begin to discharge. The presence of these electrical discharges is an indicator of hidden defects which, if left unattended, can lead to system failure. In fact, the discharges themselves will, over time, degrade the material that is sustaining them also leading to system failure. Because these discharges may occur within the interior of an insulating material and because these discharge events can be very small in absolute magnitude, their presence can be unnoticeable to human senses.
A partial discharge (PD) is a flow of electrons and ions which occurs in a gas over a small volume of the total insulation system. The defects concentrate the electric stress and can degrade the dielectric. The degradation initiates within a limited portion of the dielectric and typically does not lead to acute failure immediately. Instead, the PD in an insulation system performs as a chronic symptom which develops and eventually causes failure. The characteristics of a PD are “health indicators” which represent the performance of the insulation system in an electrical apparatus. Attempts to detect, analyze, and locate PDs have been made for a long time, including systems designed to measure and analyze acoustic energy emitted by PD pulses, optical energy emitted by PD pulses, and/or electromagnetic energy emitted by PD pulses. In electromagnetic detection systems, induction coupled (magnetic) sensors and capacitive coupled (electrical) sensors have been used.
In electromagnetic terms, a PD pulse is characterized by a fast rise time in the range of sub-nanoseconds (sub-ns) and a signal bandwidth up to the gigahertz (GHz) range at its origin. The initial ns or sub-ns spike is followed by an “ionic portion,” which may have a longer duration (about 100 ns). In a shielded power cable, the PD signal propagates along the power cable from its origin, but high frequency components of the signal attenuate more significantly (i.e., over shorter distances) than lower frequency components. Sometimes the signal bandwidth at a detection point is taken as a measure of the distance of propagation in an attempt to locate the origin of the PD.
Aspects of PD measurements are described in:                “Fundamental Limitations in the Measurement of Corona and Partial Discharge,” IEEE Transactions on Electrical Insulation, Vol. EI-17, No. 2, April 1982;        “Partial discharge detection in cables using VHF capacitive Couplers,” IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 10, No. 2, April 2003;and the references cited therein.        